A real-time interlace adjustment accounts for position errors from prior print passes to determine subsequent print pass positioning and improve output imaging.
For some inkjet printing applications, it is necessary to interlace multiple passes of the print head to achieve a higher effective printing resolution than the print head alone is capable of. For example, as shown in FIG. 1, a simple print head 100 with three exemplary nozzles 110A-C having a uniform spacing of D, can achieve an output image resolution of (1/D) if implemented in a fixed position. However, if the print head is allowed to make multiple passes in printing the output image, higher resolutions are achievable. As an example, take the case where (1/D) is equal to 150 dpi. In this example, the print head could be indexed to two print locations to achieve an output resolution of 300 dpi. In order to print a solid color patch at a print resolution of (2/D) (i.e., 300 dpi), the print head must make two passes, printing one half of the image in each pass. However, in order to achieve a uniform solid color patch at the desired output image resolution of (2/D), it is desired to place the pixels from the second pass of the print head exactly halfway between the pixels from the first pass (i.e., offset laterally from the first pass by a distance D/2).
The result of the first pass is shown in FIG. 2, with the first pass print droplets being illustrated with gray solid form. The positioning of a “desired” second pass is shown in outline form in FIG. 3 and is achieved by indexing print head 100 laterally in direction X by a distance D/2=S and advancing the print head, or the substrate, in the process direction Y. If indexed precisely, the resultant image would appear as in FIG. 4 and would have the same effect as a print generated in a single pass using a print head with twice the resolution (2/D).
However, in such a multi-pass printing scheme, there is a likely probability of introducing defects into the resultant image due to positioning errors in the motion of the print head. Specifically, assume that the printing application requires the print head to release ink at N fixed locations in the cross-process direction in order to build the final image. These desired printing locations for the system are known a priori (Xp1, Xp2, . . . , XpN). Thus, one may assume that the resultant image would always appear as shown in FIG. 4. However, there is always some amount of positioning error in the motion of the print head from one print location to the next. These errors (e1, e2, . . . , eN) cause the spacing between consecutive dots on the paper to become non-uniform, possibly leading to noticeable defects in the output image.
FIG. 5 shows the error differences between desired and actual positioning of the second pass printing, with the actual second pass printing also being shown in gray solid form. This results in the output image shown in FIG. 6. Note in this figure that the resultant image does not look uniform as it was intended to. In fact, some columns of pixels are spaced too closely, while others are separated too far apart. These defects appear as light and dark streaks in the output image. The fundamental spatial frequency of these defects can be inferred from the figure and is equal to the inverse of the nozzle spacing (1/D):ferror=1/D(cycles/mm),and is independent of the number of passes used. Note that harmonics of this fundamental frequency may also affect the resulting print quality.
An example of the typical light and/or dark bands in the print resulting from such positional error is shown in FIG. 7, which is reproduced using a more typical print head resolution. In FIG. 7, the top half was printed as a reference image with no positional errors, while the bottom half is shown with an induced 100 micron positional error to simulate the effect positional error has on the resultant image. Notice the strong periodic streaking effect on the lower image.
In the past, the standard approach to minimizing the effects of such positioning errors was to implement a scheme to reduce the magnitude of the positioning errors themselves. In most cases, electronic sensors and actuators were combined to implement control algorithms to improve the positioning accuracy of the mechanism that moves the print head. These types of schemes attempted to minimize the position error that was present at each of the printing locations independent of the positioning errors at prior print locations. That is, these methods calculated target positions for all print passes in advance of printing and tried to minimize the positional error by controlling the printing to occur as close as possible to the desired target position. Other systems used high quality/expensive mechanical positioning systems in an effort to improve positioning accuracy. There have also been prior systems that look at offline optimization schemes that are meant to adjust the calculation of the a priori positions before printing based on measured characteristics of the print head, for example (e.g. manufacturing defects).
However, such schemes can never provide perfect positioning of the print head. In other words, there will always become amount of residual error in the positioning of the print head at each location.
In addition, because of the human eye's ability to perceive the spatial frequency content of an image, these types of correction schemes do not necessarily optimize the overall perceived output image quality. In fact, it is not only the raw magnitude of the position errors themselves that determines the level of defect in the image, but also the resultant frequency content. Thus, smaller positioning errors (relative to the a priori desired print locations) on subsequent moves (given that there are errors in prior moves) do not always correspond to less noticeable defects in the output image.